When my unit (old 2.1 version) stopped working i had to figure something. With no advise how to connect existing zone expader directly to raspberry https://opensprinkler.com/forums/topic/connecting-zone-expander-directly-to-raspberry/ I figured out how to do it. Works fine for a year. I liked to had raspberry powered by POE so added module whitch transorms 48v to 5v. Of course additional 24v power supply is needed. It has rain sensor terminal as well. For additional zones – more expansion board can be connected or there is still option for relays connected directly to GPIO.